About thirty years ago it became apparent that household laundry detergents made of branched alkylbenzene sulfonates were gradually polluting rivers and lakes. Solution of the problem led to the manufacture of detergents made of linear alkylbenzene sulfonates (LABS), which were found to biodegrade more rapidly than the branched variety. Today, detergents made of LABS are manufactured worldwide.
LABS are manufactured from linear alkylbenzenes (LAB). The petrochemical industry produces LAB by dehydrogenating linear paraffins to linear olefins and then alkylating benzene with the linear olefins in the presence of HF. This is the industry's standard process. Over the last decade, environmental concerns over HF have increased, leading to a search for substitute processes employing catalysts other than HF that are equivalent or superior to the standard process. Solid alkylation catalysts, for example, are the subject of vigorous, ongoing research.
To date, alkylation processes that use catalysts other than HF, that is, commercially available solid alkylation catalysts, tend to operate at a higher molar ratio of benzene per olefin than processes that employ HF. As an illustration, while detergent alkylation processes that use HF tend to operate at a benzene/olefin molar ratio of 12:1 to 6:1, alkylation processes that use commercially available solid alkylation catalysts tend to run at higher benzene/olefin ratios, typically 30:1 to 20:1. One reason for this is that solid alkylation catalysts tend to be less selective toward producing monoalkylbenzene, and therefore the benzene/olefin molar ratio must be increased to meet increasingly stringent selectivity requirements. Selectivity, which is often defined as the weight ratio of monoalkylbenzene product to all products, is expected in some areas to be 85-90% near term, increasing to 90-95% by about the year 2000. Incidentally, a higher benzene/olefin ratio not only tends to increase selectivity but also often produces other benefits for solid alkylation catalysts, including improving olefin conversion, monoalkylbenzene linearity, and catalyst life.
As desirable as solid catalyst may be as an alternative to liquid HF, it is commonly the case that these catalysts deactivate with use. Alkylation processes, with either HF or substitute catalysts for HF, are subject to catalyst deactivation. Whereas an alkylation process employing HF typically employs an HF regenerator, an alkylation process employing a substitute catalyst such as a solid alkylation catalyst typically includes means for periodically taking the catalyst out of service and regenerating it by removing the gum-type polymers that accumulate on the surface of the catalyst and block reaction sites. For a solid alkylation catalyst, therefore, the catalyst life is measured in terms of time in service at constant conversion between regenerations. The longer the time between regenerations, the more desirable the catalyst and the process.
Although all catalysts lose some portion of their activity with continued use, the solid catalysts used to date in aromatic alkylation tend to deactivate rather quickly. Although the deactivation can be retarded by increasing alkylation reaction temperature, raising the reaction temperature tends to decrease product linearity, which is an undesirable outcome. Conversely, lowering the reaction temperature increases product linearity, a desirable result, but exacerbates catalyst deactivation leading to short useful catalyst lifetimes. As has already been suggested, catalyst life can be increased by operating at higher benzene/olefin ratio, but raising the benzene/olefin ratio increases the cost of building and operating an alkylation process, in terms of both investment capital and utilities costs. Thus, it is clear that solid catalyst can be best used in the continuous alkylation of aromatics only where effective and inexpensive means of catalyst regeneration are available.
Solid catalysts used for alkylation of aromatic compounds by olefins, especially those in the 6 to 20 carbon atom range, usually are deactivated by by-products which are preferentially adsorbed by the catalysts. Such by-products include, for example, polynuclear hydrocarbons in the 10 to 20 carbon atom range formed in the dehydrogenation of C.sub.6 to C.sub.20 linear paraffins and also include products of higher molecular weight than the desired monoalkyl benzenes, e.g., di- and tri-alkyl benzenes, as well as olefin oligomers. Although it can be readily appreciated that such catalyst deactivating agents or "poisons" are an adjunct of aromatic alkylation, fortunately it has been observed that deactivating agents can be readily desorbed from the catalyst by washing the catalyst with the aromatic reactant. Thus, catalyst reactivation, or catalyst regeneration as the term is more commonly employed, is conveniently effected by flushing the catalyst with aromatic reactants to remove accumulated poisons from the catalyst surface, generally with restoration of 100% of catalyst activity.
Therefore, it is imperative to have means of repeatedly regenerating these catalysts, i.e., to restore their activity, in order to utilize their catalytic effectiveness over long periods of time. It is further desirable to minimize the additional equipment required for regeneration and that is not used for normal operation, that is for the production of alkylaromatics. That is, one desires that any equipment that is used for alkylaromatic production be capable of a dual use for alkylation catalyst regeneration.
It is still further desirable, when converting a process unit from HF catalyst to solid alkylation catalyst, to maximize the use of equipment that exists and is in use in the HF alkylation process. Therefore, it is useful to briefly review one configuration of an HF detergent alkylation unit that gained wide acceptance during the 1970's and 1980's. That configuration uses an HF alkylation reaction section, an HF regeneration section, and an HF sludge treatment section. It is not necessary here to describe these three sections in detail, but one of the compelling reasons for switching from HF to a solid catalyst is that building and operating these three HF-containing sections have often proven to be troublesome, complex, and expensive. In addition to these three sections, this HF detergent alkylation unit also uses a series of five product recovery columns to produce a monoalkylbenzene product stream from the alkylation reaction effluent, which contains not only monoalkylbenzene but generally also benzene, paraffins, by-products, and HF. The first of the five product recovery columns is usually called an HF stripper, which strips HF from the alkylation reaction effluent for recycle to the HF alkylation reactor. The second column is generally called a benzene column, which is a distillation column that removes benzene from the HF stripper bottom stream as an overhead stream which is recycled to the HF alkylation reactor. Then, the remaining hydrocarbons flow to a series of three distillation columns: a paraffin column which removes the paraffins as a sidecut for recycle to a paraffin dehydrogenation unit if present, an LAB rerun column which removes LAB from the paraffin column bottom stream and produces an overhead stream containing the LAB product, and a heavy alkylate rerun column that removes heavy alkylate by-products including polyalkylbenzenes.
Changing from HF to a solid catalyst has greatly diminished the utility of this five-column product recovery train in existing HF detergent alkylation units, particularly in two aspects. First, the change to a solid catalyst eliminates the need for HF stripping, thereby rendering the existing HF stripper redundant. Second, the higher benzene/olefin molar ratio (e.g., 20:1 as opposed to 8:1) in the alkylation reactor more than doubles the flow of benzene to the existing benzene column, thereby flooding the existing benzene column. Too small for the higher recycle benzene flow, the existing benzene column must be replaced or supplemented with an entirely new benzene column, which greatly increases the capital cost of converting from HF to solid catalyst. But, even if a new benzene column was not needed, the operating costs of the now-converted solid catalyst unit would be much higher, because of the additional cost of the energy required to distill and condense the larger quantity of excess benzene from the alkylation reaction effluent.
Accordingly, an integrated continuous alkylation process with a method of removing catalyst deactivation agents or minimizing catalyst deactivation is sought. Such a process can increase the usefulness of commercially available solid alkylation catalyst and will help avoid the need for using HF in detergent alkylation processes.